This invention relates to the electrodeposition of chromium onto zinc or zinc alloys.
A great number of articles, such as machine parts, are fabricated from zinc base metal compositions such as zinc and zinc alloy die castings. Such articles usually are provided with surface finishes which inhibit, reduce, or eliminate corrosion associated with outdoor exposure. A commonly utilized surface finish for these purposes is a composite coating of copper, nickel, and chromium which is applied by first polishing, degreasing and cleaning the surface of the article and thereafter sequentially electrodepositing thereon layers of copper, nickel, and chromium. However, upon prolonged exposure to the elements the electrodeposited surface finishes begin to blister and peel off.
Die casting of zinc base metal is an extremely versatile method of fabricating complex metal shapes with close dimensional tolerances and at a relatively low cost. However, because of susceptibility to corrosion, a protective coating is usually required. Typical such protective coatings are applied by electrodeposition by first applying a copper strike, and then one or more coating of nickel followed by a coating of chromium. While die cast parts produced from zinc base metals provide the basic advantages of cost and weight, corrosion nevertheless is a problem even with the protective coatings. Besides, the wear resistance of such parts is notoriously inadequate for applications involving friction contact between moving surfaces.
The usual commercial chromium plating electrolytes utilized for protective coatings consist of aqueous chromic anhydride (CrO.sub.3) solutions, also commonly referred to as chromic acid solutions, which contain certain catalysts which enable the chromium contained in the solution to be electrodeposited. These catalysts usually are sulfate (SO.sub.4.sup..dbd.) and silicofluoride or fluosilicate (SiF.sub.6.sup..dbd.) ions. In order to optimize the chromium electrodeposition conditions these catalysts must be present in certain specific relative amounts based on the concentration of chromic acid present in the electrodeposition bath. In the so-called self regulating electrodeposition baths concentrations of the cooperating catalyst ions are controlled automatically by means of the solubility characteristics of the compounds that are used to supply these ions to the bath solution. Illustrative self-regulating chromium electrodeposition baths are disclosed in U.S. Pat. No. 2,640,022 to Stareck and in U.S. Pat. No. 2,686,756 to Stareck et al.
U.S. Pat. Nos. 4,095,014 and No. 4,156,634 to losso describe an effective surface treatment of zinc alloys to provide a wear resistant surface. Recent advances in zinc alloy technology, however, such as the high performance ternary zinc-copper-aluminum alloys, require adjustments in electrodeposition techniques and electrodeposition baths for the development of optimum surface properties. To that end, the present invention provides an improved electrodeposition bath composition well suited for the enhancement of the surface hardness of relatively, high copper, low aluminum zinc alloys.